High level of morphological disparity in avian skull shapes is associated with increased modularity. High level of morphological disparity in avian skull shapes is associated with increased modularity. (A) Microcomputed tomography (µCT) scan of a bird skull showing geometric morphometric landmarks (above, red dots on skull scan) and defined modules (beneath, coloured lines). (B) The phylogeny of bird species used for mapping shape data by squared-change parsimony. Representative, more basal, species were used as outgroups. (C) Morphospace occupied by skull shapes. The polygons show parts of the morphospace occupied by different groups revealing that Hawaiian honeycreepers (polygon shaded blue) have some of the most diversified and divergent skulls of all species studied. (D) Darwin's finches show strong covariation between the shape of the whole skull and several skull modules (red thick lines), suggesting strong integration within the skull. By contrast, Hawaiian honeycreepers show strong covariation only between the shape of the whole skull and that of the upper beak (blue thick line), indicating much greater flexibility of evolutionary change. Such a unique pattern of morphological integration in their skulls is likely to reflect an altered and much higher state of developmental modularity within the skull, which contributed to the outstanding level of disparity of skull morphology observed in Hawaiian honeycreepers. Cranial modules in D are colour-coded in the same way as in A. All images adapted and modified from Tokita et al. (2017). Arhat Abzhanov Development 2017;144:4284-4297 © 2017. Published by The Company of Biologists Ltd